Growing organisms must retain Na and K. The final regulation of Na and K homeostasis occurs in the renal CCD where, in the adult, the amiloride-sensitive epithelial Na channel (ENaC) and low-conductance K-secretory (SK) channel constitute the rate-limiting steps for Na absorption and K secretion, respectively. Patch clamp analysis of principal cells in CCDs isolated from maturing rabbits identifies an increase in number of conducting apical Na channels after the 1st wk of postnatal life; a surge in SK channel activity, however, is not observed until after the 2nd wk of life. The purpose of the present study was to correlate the functional expression of channel activity with appearance of channel proteins in the developing kidney. To this end, we examined the nephron distribution and localization of ENaC and SK channels in cryosections of kidney using polyclonal antibodies raised against the α-, β-, and γ-subunits of rat ENaC (generously provided by P. Smith and C. Canessa) and the putative rat SK channel, ROMK (kindly provided by L. Palmer). Because the available antibodies did not recognize rabbit antigens, studies were performed in rat; Northern analyses revealed similar developmental patterns of expression of ROMK mRNA in rat and rabbit kidney. The identity of principal cells was confirmed by the selective binding of anti-aquaporin (AQP)-3 antibody; thick ascending limbs of Henle (TALHs) were identified with an anti-uromucoid antibody. Immuno-fluorescence studies using the anti-α ENaC antibody showed apical staining of a majority of cells in CCDs isolated from all animals >1 wk of age. Similarly, β- and γ-subunit expression was routinely detected, albeit in a predominantly cytoplasmic distribution, after the 1st wk of postnatal life. ROMK, absent in the mid-outer cortex of 1-wk-old animals, was observed in the inner cortex and medulla of these same animals, presumably in TALHs. By 3 wks of age, ROMK was easily detected along the apical membrane of a discrete population of TALH-negative cortical structures, believed to represent connecting and collecting tubules. Staining with immunoabsorbed serum or secondary antibody alone was negative. These results suggest that the postnatal surges in activity of principal cell Na and SK channels are mediated, in part, by increases in abundance of ENaC and ROMK proteins in the maturing CCD.